The present invention relates generally to rotating electric machines, such as high speed generators and motors, and more particularly to magnetic structures used in rotating electric machines which are shaped to include a plurality of coil-retaining slots that are separated by radial teeth.
In rotating electric machines, such as high speed generators and motors, groups of interconnected conductive coils are commonly wound onto a magnetizable core. To assist in the coil winding process, the magnetic core is often shaped to include a plurality of slots that are separated by radial teeth, or fingers. During assembly, a plurality of the conductive coils is packed within each slot in the magnetic core.
One well known version of a magnetic core used in rotating electric machines includes a plurality of radial teeth which are generally rectangular in transverse cross-section. Because the distance, or spacing, between the distal ends of adjacent teeth is roughly equal to the distance between the proximal ends of said teeth, the magnetic core is commonly referred to in the art as having an open slot design. As can be appreciated, an open slot design enables winding coils to be readily inserted into each slot along either a radial path (i.e., at a right angle relative to the longitudinal axis of the core) or an axial path (i.e., in parallel with the longitudinal axis of the core). Once packed within the slot, the coils are held fixed in place by wedging at least one non-conductive block (commonly referred to in the art as a slot stick or wedge) in the slot between the distal ends of the adjacent pair of teeth. In this manner, the coils are less susceptible to vibrate.
Although well-known in the art, a magnetic core which utilizes an open slot design suffers from a notable drawback. Specifically, it has been found that an open slot design can significantly compromise the overall performance of the machine (e.g., power efficiency). In particular, it has been found that electric machines which include a magnetic core with an open slot design often suffer from, among other things, significant increases in slot harmonics, magnetic flux leakage and cogging torque (i.e., vibration), which are all highly undesirable.
Accordingly, in order to improve performance of the machine, it is widely known in the art to reduce the spacing between the distal ends of adjacent teeth (this spacing being commonly referred to in the art as the slot gap). Specifically, the distal end of each radial tooth in a magnetic core is often provided with a tooth tip (also referred to in the art simply as a tip) which is substantially wider than the remainder of the tooth, thereby providing the magnetic core with a closed or semi-closed slot design. One well-known version of a tooth tip includes a pair of acute corners which are rough mirror images of one another. As such, the aforementioned tooth tip provides the distal end of a magnetic core tooth with the shape of an equilateral triangle with its apex removed therefrom.
As can be appreciated, widened tooth tips provide the magnetic core of an electric machine with the ideal configuration for magnetic purposes. Specifically, the magnetic lines of flux that conduct through the core are optimized so as to lower the effects of slot harmonics, magnetic flux leakage and cogging torque, which is highly desirable. Furthermore, it should be noted that widened tooth tips also provide support for the slot sticks, thereby reducing the mechanical stress exerted thereon.
Tooth tips of the type described in detail above are often integrally formed onto the distal end of each radial tooth. Although functionally beneficial for the reasons noted in detail above, it has been found that integral tooth tips introduce a notable shortcoming in connection with machine manufacture. Specifically, because integral tooth tips significantly reduce the slot gap between adjacent teeth, it has been found to be rather difficult, if not impossible, to insert magnetic coils into each slot along a radial path. Rather, winding coils can only be inserted into each slot along an axial path, thereby complicating the coil packing process, which is highly undesirable.
Accordingly, it is well known in the art for tooth tips of magnetizable material to be removably mounted onto the distal ends of magnetic core teeth. As can be appreciated, the ability to remove each tip from its corresponding tooth enables magnetic coils to be radially disposed within each slot, thereby facilitating the coil packing process, which is highly desirable.
In U.S. Pat. No. 6,611,076 to J. R. Lindbery et al. (hereinafter referred to as the '076 patent), there is disclosed a tooth tip for a high speed generator. The tooth tips, which fit over teeth shaped into the distal ends of fingers formed between slots in laminated disks making up a portion of a stator in a generator, (i) facilitate slipping stator windings into slots from a central opening in the laminated disks, (ii) cooperate with wedges to affix stator windings in the slots; and (iii) provide a slot gap between adjacent tooth tips which is substantially smaller than the width of the slots, thereby reducing slot harmonics and improving the performance of the generator.
Although well known in the art, removable tooth tips of the type described in the '076 patent suffer from a couple notable shortcomings.
As a first shortcoming, tooth tips of the type disclosed in the '076 patent have been found to experience both (i) a relatively high level of mechanical stress as a result of the relative sharpness of its cuts (i.e., angles of curvature) along its mating interface with the core tooth and (ii) a relatively weak connective force with the core tooth due to the limited contact area established therebetween. As a result, it is required that the tooth tip be manufactured out of a relatively strong material, such as steel, to preserve its mechanical integrity. Otherwise, the tooth tip is susceptible to either fragmentation or complete separation from the magnetic core tooth during operation of the machine. As can be appreciated, the requirement that each tip be constructed using strong materials limits machine manufacturers from considering weaker materials that are considerably less expensive, which is highly undesirable.
As a second shortcoming, tooth tips of the type disclosed in the '076 patent can only be mounted on magnetic core teeth along an axial path (i.e., parallel to the longitudinal axis of the core) and not along a radial path (i.e., at a right angle relative to the longitudinal axis of the core). Consequently, if the winding coils (and wedge) are packed too tightly within a slot, axial installation of the tip on the magnetic core tooth can not be readily achieved. To the contrary, if the tip could be installed along a radial path, the tip itself could be used to impart an inward force on the winding coils to render them more compact. As a result of this limitation, the assembly of electric machines using removable tips of the type described in the '076 patent typically requires that least one tip be mounted on a tooth before packing the copper coils (and wedge) within its adjacent slot (i.e., to help retain the coils within the slot). As can be appreciated, this rearrangement of the steps of assembly substantially reduces the maximum number (i.e., density or slot fill percentage) of coils that can be packed within each slot which, in turn, limits the performance characteristics of the machine, which is highly undesirable.